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Full text of "Handbook Of Chemical Engineering - I"

EVAPORATION
395
heating shelves, upon which the ribbon is gliding. Such a dryer is extremely useful for the handling of sticky materials that have to be. dried at low temperature. The work is continuous and the operation requires little attention and labor.
A vacuum dryer is shown in Fig. 18, and the construction is substantially the same as explained under the atmospheric drum dryer, except that the housing must be very substantial and air-tight. This type of dryer while rather expensive in first cost is economical in operation, and has given excellent results for the drying of milk and delicate extracts. In all vacuum dryers, the steam consumption is about 1.2 Ib. per pound of water evaporated, and they have therefore a very much higher efficiency than the standard steam or hot air dryers.
Whatever type of dryer is used, it should be borne in mind that the material should be distributed over the heating surface evenly in small pieces and thin layers. Bulky material and large lumps will greatly retard the drying process, and while for instance a 2-in. board can be dried properly in from 2 to 3 days, it takes from 8 to 10 days to dry an 8-in. timber. The heat should be applied uniformly from all sides,
						
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FIG. 37.—Ribbon dryer (vacuum).
FIG. 38.—Vacuum drum dryer.
and the saturated vapors should be removed as quickly as possible in order that conditions may be as far as possible from equilibrium.
The drying process does not proceed at a uniform rate.    Usually the first 80 per cent of the moisture can be removed in one-fourth to one-fifth of the total drying time,